1. Field of the Invention
The present invention relates to a semiconductor amplifier, and particularly to an improvement in the efficiency of a semiconductor amplifier suitable for use in a satellite communication system, a ground microwave communication system, a radar system and the like, and which is operable in a microwave or semimicrowave frequency band.
2. Description of the Prior Art
FIG. 1 is a circuit diagram showing a conventional semiconductor amplifier which is disclosed in, for example, "SEMICONDUCTOR CIRCUIT FOR HIGH-FREQUENCY POWER", Japanese Patent Laid-Open No. 159002/1983. Referring to FIG. 1, there are shown an input terminal 1, an output terminal 2, an FET (Field Effect Transistor) 3, a gate terminal 4, a drain terminal 5, a source terminal 6, a gate bias terminal 11, a drain bias terminal 12, a gate bias choke line 13 used to apply a bias to gate 4, a drain bias choke line 14 used to apply a bias to drain 5, an input-side d.c. blocking capacitor 16, an input-side impedance matching line 17, an output-side impedance matching line 18, an input-side impedance matching capacitor 19, an output-side impedance matching capacitor 20, a second harmonic or second order signal short-circuiting line 21 and a capacitor 22.
The second harmonic signal short-circuiting line 21 has a length corresponding to about 1/4 wavelength of a fundamental wave. Line 21 is a strip line or distributed constant line that is conventionally used in microwave band circuits.
A description will now be made of the operation of the conventional semiconductor amplifier.
The conventional semiconductor amplifier is operated at a high efficiency because the second harmonic signal (i.e., a signal having twice the frequency of the input signal) is canceled or short-circuited at the drain terminal 5 by means of the short-circuiting strip line 21 and the capacitor 22. Since the gain of the FET transistor 3 must be set at the design stage to be precisely controlled, the amplitude of the second harmonic signal is determined by the amplitude of the fundamental signal. Although the FET produces multiple harmonics of the input signal, harmonics above the second have a very small amplitude and can thus be neglected.
As described above, the conventional semiconductor amplifier serves to short-circuit the second order signal by making use of the second order signal short-circuiting strip line 21 and the capacitor 22 as a passive circuit. However, the semiconductor amplifier is accompanied by the problem in that the second harmonic signal cannot be short-circuited without failure in a microwave band because the second harmonic signal short-circuiting strip line 21 and the capacitor 22 are subject to great losses within this band.